The present invention relates to an optical pickup in which an optical element having a function for increasing the numerical aperture, for example, a SIL (Solid Immersion Lens) or the like, is disposed close to a recording medium to perform recording and/or reproducing of information on the recording medium at a high density.
Japanese Patent Publication No. 2,553,275 discloses an optical system which is capable of performing recording and/or reproducing of information on a recording medium. In this conventional optical system, converging light from an objective lens is further converged through a semi-spherical SIL so that a diameter of a spot formed on a recording layer of the recording medium is reduced. The semi-spherical SIL increases the numerical aperture of the optical system by a multiple corresponding to its own refractive index.
In an information recording and reproducing technique of the optical system, when information is recorded on the recording layer of the recording medium and it is reproduced from the recording layer, it is necessary to put the semi-spherical SIL in much close to the recording layer. The distance from the semi-spherical SIL to the recording layer is significantly small as compared with the distance from an objective lens to the recording layer in a conventional information recording and reproducing technique of another kind in which light converged by only the objective lens is projected through a front side surface of such a recording medium as a CD (Compact Disc) or a MO (Magneto-Optical Disc) onto a recording layer on a back side surface thereof to perform recording and reproducing of information on is the recording layer. For this reason, the recording and reproducing technique using the SIL is called as a near field recording and reproducing technique, while the recording and reproducing technique using only the objective lens is called as a far field recording and reproducing technique.
Nikkei Electronics, Pages 99 to 105, issued on Jun. 16, 1997 (No. 691) has disclosed improvement for such a near field recording and reproducing technique. In the improved near field recording and reproducing technique, a super semi-spherical SIL or a stigmatic focusing SIL is used instead of the semi-spherical SIL. The super semi-spherical SIL or the stigmatic focusing SIL increases the numerical aperture of an optical system using the same by a multiple equal to the second power of the refractive index thereof.
Japanese Patent Application KOKAI Publication No. 8-315404 has disclosed an optical pickup having an optical system in which an objective lens and a generally semi-spherical SIL are combined. The optical pickup records information on and reproduces it from a recording surface in a state where the generally semi-spherical SIL is brought in much close to a thin cover glass which covers the recording surface of the recording medium.
In the plurality of the conventional techniques described above, when recording and/or reproducing of information is performed on the recording layer of the recording medium by using the SIL, it is necessary to bring the SIL in much close to the recording layer. In a typical example, the SIL is brought in close to the recording layer up to at least 500 mμ or less, or 200 nm or less. Incidentally, the numerical aperture increased by a lens with a high numerical aperture is at least 0.65 or more. In recent years, a technique for increasing the numerical aperture to 0.8 to 0.9 or more has been published in relevant academic meetings.
However, in the above-mentioned conventional near field recording and reproducing techniques, there are the following problems.
Though the recording layer on the front side surface of the recording medium is covered with a thin transparent protective film, contaminant such as moisture, organic materials, dusts and the like in the atmosphere adheres to the front side surface of the protective film. Accordingly, when the recording medium is rotated in order to perform recording and/or reproducing of information on the recording layer on the front side surface of the recording medium in a state where the SIL has been brought in much close to the recording surface on the front side surface of the recording medium, contaminant is scattered by a centrifugal force acting thereon and the contaminant is adhered and deposited on the SIL. Particularly, the contaminant which has been scattered from the protective film of the recording medium is adhered and deposited on a facing surface of the SIL facing the protective film. When the adhesion and deposition of such contaminant on to an optically effective area on the facing surface of the SIL become heavy, the transmittance of laser beam on the optically effective area is gradually decreased (the optical performance of the SIL is gradually lowered), so that it becomes difficult to record information on and/or reproduce it from the recording surface of the front side surface of the recording medium for a long time of period.